Some fans of Britney Spears are accusing the singer of lip-syncing to her song "Perfume" during her Las Vegas show. According to gossip site Oh No They Didn't, Spears' mouth is moving but the audience is mostly hearing singer Sia's version of the track.

Billboard points out that Sia -- best known for her hit song "Chandelier" -- is credited as one of the writers of "Perfume." An acoustic version of the song as performed by Sia is big on YouTube.

"If you listen closely to the performance video, you can make out some of Spears' vocals under Sia's more charactered voice," Colin Stutz writes for Billboard. "That's because, most likely, Spears uses Sia's track as an in-monitor guide when she sings. Only this time it was put into the house mix, making for an embarrassing accident."

It's not the first time Spears has been called out for the vocalizations during her Vegas show, which is part of a two-year residency. In reviewing the show for the Los Angeles Times, writer Mikael Wood points out that the singer's show appears to take its title, "Piece of Me," literally.

"Whatever the scale of the number ... the singer's presence felt so diminished -- her dancing a tentative shadow of what it used to be, her vocals apparently lip-synced for the majority of the show -- as if to make the production's title seem a taunt," Wood writes.

While her manager Adam Leber initially denied that Spears wasn't singing live during the show, he later backtracked during an interview with Medium.com.

"To put on the show that she puts on, it's virtually impossible to sing the entire time and do what she does," he said.

"She's singing on every song, basically, when she has the ability to sing. There's no way you can dance for 90 minutes straight and sing the entire time."